In a large number of situations it is desirable to be able to analyze a sample, be it a liquid sample or a gaseous sample, for one or more constituents. Often, it is desirable to analyze for several constituents at once. For example, it is desirable to be able to analyze blood for such diverse components as H.sup.+, K.sup.+, CO.sub.2 and O.sub.2, etc. It is also often desirable to be able to analyze air samples for air borne contaminants such as CO, NO, NO.sub.2, N.sub.2 O, SO.sub.2, H.sub.2 S and O.sub.2 and other gases as well.
Within the last several years a number of sensors have been developed based upon one or more of the techniques developed by integrated circuit engineering technology. For example, U.S. Pat. No. 4,020,830, issued May 3, 1977 to C. C. Johnson, et al, utilizes a chemical sensitive field-effect transistor (FET) transducer for selectively detecting and measuring chemical properties of substances to which the transducer is exposed. Basically, the chemical being detected interacts with certain substances to modulate the electric field produced in the substrate semiconductor material between diffusion regions thereof. Such FET devices have been demonstrated to be useable for detecting ions as well as gases, and indirectly certain dissolved molecules. However, fluctuations in drain current leading to errors are still a significant problem. These fluctuations can be associated with thermal noise or they can be light induced. Layers that make the FET chemically sensitive and selective are very difficult to deposit on the gates of such devices, especially since often several layers of different composition are needed. All of this leads to errors or makes fabrication difficult. Still further, reference electrodes are very difficult to implement in FET structures.
S. J. Pace, as set forth in U.S. Pat. No. 4,225,410, discloses a disposable integrated miniaturized array of chemical sensors for analyzing concurrently a number of analytes in a fluid sample. Each sensor of the array is a complete electrochemical cell having its own reference and indicator electrodes and is selective with respect to a particular analyte. The sensors are all formed on top of the surface of a substrate which is prepared by press forming powdered alumina with appropriate through holes and imprints for the electrochemical circuit. Because of the manufacturing techniques such sensors and sensor arrays must be relatively large and are more properly describable as minisensors rather than microsensors.
In U.S. Pat. No. 4,549,951, issued Oct. 29, 1985 to M. B. Knudson, et al, a relatively large, compared to both of the devices discussed above, ion selective electrode is set forth which is used along with a separate reference electrode. The ion-selective membrane of the electrode sits on a conductor embedded in a plastic substrate, This is basically a small ion-selective electrode with the membrane sitting on top of a conductor and without an internal reference electrolyte or true reference electrode. Further, construction of such an electrode design in micro sizes appears to be beyond the current state of the art.
In the devices of U.S. Pat. Nos. 4,020,830, 4,225,410, and 4,549,951 the entire electrochemical cell sits upon the surface of a substrate. This leads to a significant problem in providing proper encapsulation. In the case of U.S. Pat. No. 4,020,830, all of the electronic circuitry is included on the analyte detecting side of the FET. This leads to problems between the chemicals and the electronic circuitry which are either in contact with one another or closely adjacent to one another.
The prior art, including the above discussed patents, does not yet provide microelectrochemical sensors and sensor arrays incorporating both amperometric and potentiometric elements, which operate at room temperature and consume little power, which provide versatile, multi-purpose-multi-channel, real time monitoring of vapors, gases, molecules and ions, which are micro-portable and field rugged, which have fast response times at ambient temperature, which are free of interferences from such parameters as oxygen deficiency and humidity, which can be produced inexpensively using sophisticated modern micro-fabrication technologies, which have high specificity and high selectivity, for example, parts-per-billion level detection of such gases CO, NO, NO.sub.2, H.sub.2 S, SO.sub.2, and N.sub.2 H.sub.4 and parts-per-million detection of such gases as HCN, Cl.sub.2, H.sub.2, O.sub.2, C.sub.2 H.sub.5 OH, HCHO, C.sub.3 H.sub.3 N, O.sub.3, C.sub.2 H.sub.2, C.sub.2 H.sub.4, CH.sub.4, C.sub.2 H.sub.6, C.sub.3 H.sub.8, and organophosphate vapors, and which are adaptable for detecting ionic electroactive species in parts-per-billion in solutions, including, for example, Cl.sup.-, Br.sup.-, I.sup.-, SCN.sup.-, CN.sup.-, S.sub.2 O.sub.3.sup.2-, OCl.sup.-, SO.sub.3.sup.2-, phenols, aromatic amines, nitro compounds, organoarsines, and metal ions, e.g., Cu.sup.2+, Fe.sup.3+.
The present invention is directed to solving one or more of the problems as set forth above.